Tsaur et al U.S. Pat. Nos. 5,147,771, 5,147,772 and 5,147,773 disclose the use of varied polyalkylene oxide block copolymer surfactants in the precipitation of radiation-sensitive tabular grain silver halide emulsions useful in photography. The polyalkylene oxide block copolymer surfactants are disclosed to be useful in reducing the proportion of unwanted grain shapes and decreasing grain dispersity, both in grain projected area and grain thickness. Tsaur et al U.S. Pat. No. 5,171,659 discloses the capability of polyalkylene oxide block copolymers to reduce the coefficient of variation of grain equivalent circular diameter to less than 10 percent. Numerous subsequent patents have applied the teachings of Tsaur et al to the preparation of silver halide emulsions for varied photographic and radiographic applications.
Although Tsaur et al dispersed the polyalkylene oxide block copolymers directly in the aqueous dispersing medium of silver halide precipitation reaction vessels, disadvantages with this approach have been observed. The high viscosity of the polyalkylene oxide block copolymers makes reproducibly accurate introductions of small amounts of the block copolymers difficult. Additionally, getting the block copolymers to disperse uniformly within the aqueous dispersing medium is difficult. Block copolymer dispersion non-uniformities are a significant contributor to batch-to-batch variations in emulsion characteristics.
When an addendum necessary to the preparation of a radiation-sensitive silver halide emulsion before, during or after grain formation does not disperse adequately in water, a common practice is to disperse the addendum in a water miscible organic solvent, such as acetone, methanol, ethanol, propanol, 2-methoxyethanol or 2-ethoxyethanol, prior to or during addition to the aqueous dispersing medium of the emulsion or in which the emulsion is to be formed. Illustrations of water miscible organic solvents used in silver halide emulsion preparation in combination with polyalkylene oxide block copolymers are provided by Goan U.S. Pat. No. 5,420,007, Takahashi U.S. Pat. No. 5,468,602, Ikeda et al U.S. Pat. No. 5,587,280 and Brust et al U.S. Pat. No. 5,763,151.
Unfortunately, the presence of water miscible organic solvents in emulsions can cause undesirable interactions with other components added during or subsequent to emulsion precipitation. For example, emulsions commonly include combinations of coating aids, coupler solvents and vehicle modifiers, and many of these materials can enter into unwanted interactions with one or more water miscible organic solvents resulting in emulsion coatings that are either physically degraded (e.g., non-uniform) or deficient in one or more photographic performance characteristics. Reductions in surface activity, crystallization, and flocculation are examples of unwanted physical effects.
Even when water miscible organic solvents are chosen that are compatible with the emulsion physical and photographic performance properties sought, there remains the problem of dealing with volatilization of the organic solvents from the emulsions and photographic coatings. Organic vapors are dangerous and burdensome to control within the confines of manufacturing operations. Emissions of organic vapors into the atmosphere are potentially damaging to the Earth's ecology and must be limited to satisfy government environmental protection requirements.
Blease et al U.S. Pat. No. 5,460,937 discloses a method of addition of solid hydrophobic photographically useful compounds into an aqueous photographic composition that eliminates the necessity of relying on a water miscible organic solvent. Particles of the hydrophobic photographically useful material are mixed with a hydrophilic solid material. The mixture is heated to its softening point and then expelled for rapid cooling, creating a solid mixture with a specific surface area of at least about 10 square centimeters per gram. The hydrophilic solid and the high specific surface area facilitate dispersion of the hydrophobic solid within an aqueous photographic composition. Hydrophobic photographically useful materials include spectral sensitizing dyes, absorbing dyes, antifoggants, antioxidants, nucleating agents, biostats, biocides, antifoamants, development accelerators, emulsion finish modifiers, image toners, optical brighteners, couplers, antistain agents, hardeners, storage stabilizers, and latent image stabilizers. The hydrophilic solid can take the form of water soluble polymer, employed alone or in combination with water soluble lower molecular weight materials having a melting point between 30 and 200.degree. C.